1. Field of the Invention
This invention relates to a combination of a plate for exposure and frame plates which is used with an exposure device.
2. Description of the Prior Art
There is known a combination of a plate 1 for exposure having a pair of photosensitive layers 5 coated on its opposite surfaces, respectively, and a pair of transparent frame plates between which the plate 1 is disposed. The plate 1 is, for example, a printed circuit board. The plate 1 is formed therethrough with at least two positioning apertures 2 adjacent to its peripheral edge. The plate 1 and the photosensitive layers 5 are fragmentarily shown in FIG. 1 in which one of the positioning apertures 2 is also shown. One of the frame plates, or the lower frame plate is provided with a corresponding number of positioning pins on its upper surface adjacent to its peripheral edge, and mounted in an exposure device. The lower frame plate carries a mask in the form of a film or sheet on its upper surface. The plate for exposure is attached to the lower frame plate by the positioning pins fitted in the positioning apertures. The upper frame plate is also formed with the corresponding number of positioning apertures adjacent to its peripheral edge, and carries a mask on its lower surface. The pins of the lower frame plate extend through the apertures of the plate for exposure into the apertures of the upper frame plate, whereby the plate for exposure and the two frame plates are put together. The plate for exposure is exposed to the light radiated through the upper and lower frame plates.
The surfaces of the plate 1 are covered by a copper coating 4 which also covers the peripheral surfaces of the apertures 2. The thickness of the copper coating 4 lacks uniformity for various reasons; therefore, it presents a considerably uneven surface particularly in the apertures 2, as shown in FIG. 2. This uneven surface requires an excessively large force for the insertion of the positioning pins into the apertures 2 of the plate 1, since the apertures 2 and the pins are substantially equal in diameter to each other to achieve a higher degree of positional accuracy. A large force is, therefore, required for removing the pins from the apertures. When the pins are forced into the apertures 2, the copper coating 4 in the apertures 2 is partly peeled by the pins, and the dust of the copper coating 4 adheres to the surfaces of the photosensitive layers 5. This has been a major cause for the formation of a defective mask pattern for exposure.
The positioning of the plate 1 for exposure is based on the lower frame plate, as hereinabove stated. Therefore, it is often difficult to position the plate 1 accurately with respect to the upper frame plate, and it is in many cases necessary to ascertain the accuracy of their positioning visually.